Call of Duty 2 graphic issues

I'm pretty much a novice when it comes to fixing problems. I got Call of Duty 2 and put it on my computer and it ran fine, up until you start the mission. Then, the graphics become sluggish and they freeze and then go again. All those realy annoying things you deal with when there is a compatibility issue. I tried changing the screen resolution to the lowest possible (i think 680X something), and that helped a little. Basically, it's still sluggish. Any ideas? What info do I need to provide?